In the manufacture of metal cylinders such as for electric motor housings it is common to use mating mandrel and die metal forming techniques. With these techniques there often are difficulties in maintaining the squareness, circularity and diameter control which are desired at the ends of the cylinders where they engage, locate and align next assemblies, or internally where they receive and locate a next assembly. Furthermore, close control of the total cylinder length is often desirable in order to optimize assembly operations and product performance. Sometimes there also are special requirements for end counterbore diameters which are somewhat different than the main cylinder diameters, or for special non-circular features at the end of the cylinder such as axial or radial "stands" or extensions to position mating parts. Also, there are circumstances where a centrally-mating part must be accommodated by an arcuate and accurately located circumferential surface.
The ability to control length, squareness, circularity, bell-mouthing, and mating surface characteristics of fabricated cylinders is limited by the inherent characteristics of the cut metal stock, and of the die-mandrel metal forming processes. Limitations in stock-tooling alignment, and variations in the thickness and/or width of the unformed stock, and in its bending, hardness, and springback characteristics, result in variations in the formed cylinders. These variations are particularly evident at the ends and inside diameters of the cylinders.
The ends of curved elements such as cylinders are usually vital areas with respect to proper fit and alignment with mating parts. Often a second machining operation must be performed on the ends of formed metal closures, and in some cases even on cutoff tube lengths, to control the squareness or circularity at the ends. This is also true in die-forming techniques, even when the forming tools include stock edge restraint. In fact, if edge restraint clearances are tight, even slight variations in stock width or alignment can create edge burrs and misformed parts. As to accurate counterbore diameters, and non-circular edge positioning features, with known techniques they are not reasonably provided for during single stroke processes for forming curved structures. Neither are close inside diameters.
It is an object of this invention to provide improved curved structures which are "precision-reformed", and efficient diemandrel techniques for making such shapes which can uniformly produce a product which is well within common tolerance requirements of such parts. These parts are useful without requiring resort to such expedients as self-aligning bearings and special end-play spacers, or secondary operations such as machining of the ends of the structure.
In fact, with this invention, housing assemblies and other curved structures can be made with precision comparable to cavityformed parts, but they can instead be made on conventional punch press equipment. A one-stroke press operation can be made to give results at least equal to those obtained with cut-off tubular workpieces.